Toxoplasmic encephalitis is a common opportunistic infection in AIDS patients. The currently available chemotherapy with pyrimethamine plus a sulfa drug does not always prevent death. Thus new drugs are needed to treat toxoplasmosis in immunosuppressed patients. These new drugs will be sought among the many anticoccidial agents that have been designed to treat infections of chickens with Eimeria species. Two of these anticoccidials, arprinocid and emimycin, have already shown potent activity in animal experiments. Mutants of Toxoplasma gondii resistant to both these drugs have been isolated. One mutant has been used to prove that the form of arprinocid that is active in vivo is the N-oxide. The mechanism of action of emimycin and of arprinocid-N-oxide will be determined. Emimycin is likely to affect pyrimidine metabolism in the parasite while arprinocid-N-oxide may affect purine metabolism. The actions of both drugs will be studied initially by analyses of their effects on the nucleotide pool of T. gondii. Among the additional biochemical aspects to be examined will be permeability to the drug, effects of the drug on various purine and pyrimidine salvage enzymes, and incorporation of the drug into nucleic acids. The mechanisms of action of other antitoxoplasma drugs that will be identified by screening a large catalogue of anticoccidials will also be studied. A key component in analyzing these mechanisms will be the use of multi- and single-step drug resistant mutants of T. gondii. In addition, already isolated mutants of the parasite resistant to pyrimethamine will be studied by comparing the wide type and mutant dihydrofolate reductases. Similarly, already isolated mutants, of T gondii resistant to sulfadiazine will be examined by comparing the wild type and mutant guanosine triphosphate cyclophydrolases. Understanding the mechanisms of action of these various drugs should provide valuable insights into the basic biology and biochemistry of this parasite.